Method to prepare nanoparticles suspension in ionic liquids

ABSTRACT

A method to preparing suspensions of metal or metal alloy nanoparticles in an ionic liquid involves the physical vapor deposition of a metal or a mixture of metals onto an ionic liquid. The method can be modified by the introduction of a reagent during or after formation of the suspension to yield nanoparticles of a metal salt. The nanoparticles can be isolated from the suspension by the thermal decomposition of the ionic liquid under conditions where the decomposition products are gaseous.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The present invention was developed under contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The U.S. Government has certain rights in the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable

FIELD OF THE INVENTION

The invention pertains to suspensions of nanoparticles in ionic liquids.

BACKGROUND OF THE INVENTION

Ultrafine nanoparticles are particles with diameters of only a few nanometers. Metal nanoparticles are useful for applications such as catalysts, optics separations, sensors, and electronics. The typical manner in which such nanoparticles are prepared is in solution by the reduction of the corresponding metal ions or complexes, generally in the presence of stabilizing agents. The use of such a reaction method to generate the nanoparticles results in particles that include byproducts, unconverted reagents, and stabilizing additives. The presence of these impurities is often detrimental to the properties of the nanoparticles of interest in various applications, such as for catalyst where these impurities can act as catalyst poisons.

Numerous methods to synthesize nanoparticles via a chemical reaction exist. One common wet chemical approaches for synthesizing nanoparticles, such as CdS quantum dots, is known as the TOP/TOPO method (TOP=tri-n-octgylphosphine, TOPO=tri-n-octgylphosphine oxide). In this method, organometallic reagents such as dimethylcadmium are pyrolyzed by rapid injection into hot TOP and/or TOPO at 250-300° C. The coordinating nature of these solvents provides temporally discrete nucleation and permits some control over size distributions, however, there are a number of limitations using this approach and other common methods such as thiol capping groups. These limitations include precursor toxicity, limited choice of organometallic precursor, high temperature requirements, chemical impurities, and the presence of a capping layer on the nanoparticle surface. These problems are often compounded in attempts to produce metal oxide materials, for instance. A number of methods for further improving size distributions in colloidal solutions have been proposed. These involve decreasing surface tension at the liquid/nanoparticle interface and decreasing the mass transfer coefficient.

Metal-vapor-deposition techniques have been developed for the clean preparation of metal nanoparticles dispersed in organic solvents by a physical rather than chemical method. These physical methods are considered clean because no byproducts, unconverted reagents, or additives need be present that can contaminate the nanoparticles. These techniques are generally of two types, one depositing metal onto the surface of a flowing solvent of very low vapor pressure and the other by a co-condensation and freezing of the metal with a volatile solvent on an extremely cold surface.

Both variations of this vapor deposition route are illustrated in U.S. Patent Publication No. 2005/0126340 to Fujimoto. The first type of the deposition method is illustrated where a relatively non-volatile cooled oil flows via centrifugal force while one or more metals are sputtered onto the surface of the flowing oil. The surface of the oil is refreshed frequently to assure that a film is not formed, and the particles are collected from a surface on which they impinge, separating them from the oil. The second type involves the sputtering of the metals while also sputtering an organic vapor-liquid suspension onto a liquid-nitrogen cooled surface. The resulting solid suspension was allowed to warm and melt permitting the separation of the metal nanoparticle from the liquid in a manner equivalent to that of the first type using a non-volatile oil.

Ionic liquids are salts consisting of a cation and anion pair where the cation, anion, or both are of a structure that resist crystallization and are liquids at relatively low temperatures, often at room temperature and below. Generally the ionic liquidity is ascribed as resulting from a bulky, asymmetric cation. Because they are ionic, these liquids display many unique and desirable properties. Even in the liquid state, ionic liquids generally exhibit well defined long-range structural order reminiscent of surfactant solutions. They have been found to be useful for preparing nanoparticles via traditional reaction techniques. However, these nanoparticles prepared using ionic liquids also contain the reaction byproducts which should be avoided for many applications.

SUMMARY OF THE INVENTION

A method for preparing a suspension of nanoparticles includes the steps of providing one or more ionic liquids and depositing as a vapor at least one metal onto the ionic liquid to yield metal containing nanoparticles suspended in the ionic liquid. The metal containing nanoparticles can be a metal wherein a single metal is deposited. The metal containing nanoparticles can be an alloy wherein a plurality of metals is deposited simultaneously. The metal containing nanoparticles can be a mixture of different metal nanoparticles wherein a plurality of metals is deposited sequentially. The step of depositing can be carried out in an atmosphere of a reactive gas to yield nanoparticles comprise metal oxides, metal sulfides, metal nitride, metal oxysulfides, or metal oxynitrides. The reactive gas can be selected individually or in combination from the group consisting of oxygen, nitrogen, ammonia, and hydrogen sulfide. The metal can be platinum, silver, gold, cobalt, nickel, iron, manganese, rhodium, palladium, rhenium, ruthenium, iridium or osmium. The ionic liquid can have cations selected from the group consisting of 1-alkyl-3-methylimidizolium, N-alkyl pyridinium, mono-, di-, tri- or tetraalkyl ammonium, mono-, di-, tri- or tetraalkylphosphonium, and anions selected from the group consisting of Cl⁻, Br⁻, I⁻, NO₃ ⁻, BF₄ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, CH₃(CH₂)_(x)CO₂ ⁻ where x=0 to 18, and BR₄ ⁻ where R=independently C₁ to C₈alkyl. The step of depositing can be sputtering. The method can further include the step of introducing a source of oxygen, sulfur or nitrogen at a sufficiently high temperature to a suspension of metal nanoparticles where the suspension is converted into a suspension of nanoparticles of metal oxides, metal sulfides, or metal nitrides.

A method for preparing nanoparticles includes the steps of providing an ionic liquid; depositing as a vapor at least one metal onto the ionic liquid to yield metal containing nanoparticles suspended in the ionic liquid; and heating the suspension to thermally decompose the ionic liquid to gaseous neutral molecules leaving the resulting nanoparticles essentially free of the ionic liquid, decomposition products of the ionic liquid, and other impurities. The metal containing nanoparticles can be a metal when a single metal is deposited. The metal containing nanoparticles can be an alloy when a plurality of metals is deposited simultaneously. The metal containing nanoparticles can be a mixture of different metal nanoparticles when a plurality of metals is deposited sequentially. The step of depositing can be carried out in an atmosphere of a reactive gas to yield resulting nanoparticles that are metal oxides, metal sulfides, metal nitride, metal oxysulfides, or metal oxynitrides. The reactive gas can be selected individually or in combination from the group consisting of oxygen, nitrogen, ammonia, and hydrogen sulfide. The metal can be platinum, silver, gold, cobalt, nickel, iron, manganese, rhodium, palladium, rhenium, ruthenium, iridium or osmium. The ionic liquid can include cations selected from the group consisting of 1-alkyl-3-methylimidizolium, N-alkyl pyridinium, mono-, di-, tri- or tetraalkyl ammonium, and mono-, di-, tri- or tetraalkylphosphonium and anions selected from the group consisting of Cl⁻, Br⁻, I⁻, NO₃ ⁻, BF₄ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, CH₃(CH₂)_(x)CO₂ ⁻ where x=0 to 18, and BR₄ ⁻ where R=independently C₁ to C₈alkyl. The step of depositing can be sputtering.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an aberration corrected scanned transmission electron microscope (STEM) image of a suspension of 1 nm Pt nanoparticles, as white features, suspended in 1-butyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)-imide (BMIM-Tf₂N) where the reference bar is 5 nm.

DETAILED DESCRIPTION OF THE INVENTION

A method for preparing a suspension of nanoparticles in an ionic liquid comprises the physical vapor deposition of at least one metal onto a surface comprising at least one ionic liquid. Preferably the surface is in a state of constant agitation or stirring. The physical vapor deposition can be by vacuum evaporation, sputtering or other process where the metal is vaporized from a target metal and condensed on a surface. The size of the nanoparticles that are formed can be selected and controlled by the composition of the ionic liquid and/or the composition of a mixture of ionic liquids.

Ionic liquids enable the method due to special properties of these liquids. As disclosed herein, ionic liquids are defined as salts having a melting points below 100° C., and are generally those referred to as room temperature ionic liquids where they are fluid at normal room temperatures. By carrying out the nanoparticle formation by physical vapor deposition, the inclusion of reaction by-products that result during formation of metallic nanoparticles from metal salt or metal complex precursor and a complimentary reactant can be avoided.

One property of ionic liquids is that they display little or no vapor pressure below their decomposition temperature. Therefore, the deposition can be carried out without the requirement of cooling the ionic liquid. This contrasts with known methods where non-ionic liquids are used to prepare nanoparticles via a deposition process where the liquid must be cooled significantly to suppress the vapor pressure of the liquid. The ionic liquids lack of vapor pressure promotes a consistent product over a relatively large range of temperatures and exposed surface areas, and permits a consistent product over a rather large processing window and with a variety of deposition protocols. The processing window can include a relatively wide range of temperatures requiring only that the temperature is sufficiently high for the suspension to display a desired viscosity range and sufficiently low to avoid decomposition of the ionic liquid which is dependent upon the specific ionic liquid or mixture of ionic liquids. The size of the nanoparticles that are formed can be selected and controlled primarily by the composition of the ionic liquid and/or mixture of ionic liquids.

In the physical deposition process the ionic liquid is used at a desired viscosity or range of viscosities. The desired viscosity may be achieved by heating the ionic liquid as long as that temperature is below the onset of decomposition. Practical pressures for the physical vapor deposition of metals are generally from around 1 millitorr to about 50 millitorr. Rates of deposition are generally impractically slow at pressures significantly higher than 50 millitorr. The deposition rate can be generally increased for any given pressure by increasing the power generating the plasma and/or reducing the distance between the target metal and the ionic liquid surface. Those skilled in the art can readily select appropriate conditions for a given apparatus, metal, and ionic liquid to optimize the throughput for preparation of a given ionic liquid suspension of nanoparticles.

Another feature of ionic liquids exploited for use in this method is that ionic liquids can stabilize suspensions of metal particle without a surfactant or other stabilizer such that agglomeration or coalescence of the nanoparticles can be minimized or avoided entirely. By stabilizing individual particles, suspensions can be prepared where the average particle size can be very small and the distribution of particles sizes can be narrow relative to those prepared via prior art methods involving chemical transformations to form the nanoparticles. The stability permits the storage of a nanoparticle suspension in an ionic liquid for a long period of time. FIG. 1 is an aberration corrected scanning transmission electron microscope (STEM) image of platinum nanoparticles with a diameter of about 1 nanometer suspended in 1-butyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)-imide (BMIM-Tf₂N). The suspension had been stored at room temperature for 21 months prior to performance of the microscopy. Although the stability of the suspension will vary with the metal and ionic liquid used, suspensions which are stable in excess of a month are typical, with many displaying stability over a period of 6 months or more. If desired, the long term stability can be further enhanced by using an ionic liquid that is a solid at normal room temperatures but can be heated to a fluid state for the deposition of a metal and permitted to solidify around the suspended nanoparticles for storage of extremely long periods of time before use in an application where the solid suspension is fluidized by heating.

Any metal which forms a solid at normal room temperatures can be used in the practice of the method. These metals include but are not limited to platinum, silver, gold, cobalt, nickel, iron, manganese, rhodium, palladium, rhenium, ruthenium, iridium or osmium. The deposition source can be a mixture of two or more metals and the resulting nanoparticles can be an alloy.

The size of the nanoparticle suspended in the ionic liquid can be controlled by the ionic liquid used. Prolongation of deposition time generally results in a higher concentration of nanoparticles in the ionic liquid suspension, but does not cause a remarkable change in their size.

The ionic liquid is chosen to have an onset of decomposition temperature that is sufficiently high, generally being at least 20° C. greater than the temperature at which the suspension is prepared. Preferably the ionic liquid displays a melting point below about 100° C. but liquids with higher melting points can be used.

Ionic liquids that can be used in the practice of the method include, but are not limited to, those with cation structures of: 1-alkyl-3-methylimidizolium (I); N-alkyl pyridinium (II); mono-, di-, tri- or tetraalkyl ammonium (III); or mono-, di-, tri- or tetraalkylphosphonium (IV) as indicated below.

where R═C₁ to C₈ alkyl

where R═C₁ to C₈ alkyl

where R=independently H, C₁ to C₈ alkyl and where at least one R≠H

where R=independently H, C₁ to C₈ alkyl and at least one R≠H.

The anion structure of the ionic liquid can include Cl³¹ , Br⁻, I⁻, NO₃ ⁻, BF₄ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, CH₃(CH₂)_(x)CO₂ ⁻ where x=0 to 18, or BR₄ ⁻ where R=independently C₁ to C₈ alkyl. The ionic liquid can be a mixture of one or more cations with a common anion, a mixture of anions with a common cation, or a mixture of cations with a mixture of anions.

The method can be used to synthesize oxide, sulfides, carbide, nitride or mixtures of phases by the inclusion of a reactive gas to the deposition process. For example, the metal deposition can be carried out in an atmosphere of oxygen to form a metal oxide, in an atmosphere of nitrogen or ammonia to form a metal nitride, or a hydrogen sulfide atmosphere to form a metal sulfide. These gases or mixtures of these gases would react in a plasma to form the metal oxide, nitride, sulfide, oxynitride, or oxysulfide which would be deposited in the ionic liquid. In some cases, adding a reactive gas to the nanoparticle loaded ionic liquid while maintaining the suspension at an appropriate temperature below the decomposition temperature of the ionic liquid can result in the conversion of the metals into the metal oxides sulfides, or nitrides.

One feature of ionic liquids is that they can be decomposed thermally at a temperature and pressure where the decomposition products form at a temperature significantly above their boiling points. Ionic liquids often display an onset of decomposition that is not significantly higher than a temperature where catastrophic decomposition occurs. The decomposition generally occurs by reaction of the anion and cation components of the ionic liquid. Depending upon the structure of the ionic liquids the decomposition can occur via a substitution reaction, an elimination reaction, or any other disproportionation reaction to yield neutral compounds. By selecting the pressure under which the thermal decomposition is carried out, the ionic liquid can be rapidly removed as neutral gas molecules leaving only highly pure nanoparticles essentially free of the ionic liquids or their decomposition products. The pressure under which decomposition and removal of the ionic liquid is carried out can range from high vacuum to a pressure in excess of one atmosphere, This feature of the inventive nanoparticle suspensions in ionic liquids can be exploited for the placement and isolation of nanoparticles in a manner conducive to providing a nanoparticle that is sufficiently free to function for an intended purpose, such as a catalyst, where nanoparticle suspensions prepared and provided by other methods cannot readily provide nanoparticle free of surfactants and/or reaction by-products.

Not only can the resulting metal nanoparticles isolated from suspensions in ionic liquids be essentially free of non-metallic impurities, they also can display little coalescence into larger particles or aggregation of particles from that of the initially deposited nanoparticles in the ionic liquid upon decomposition and removal of the ionic liquid. The thermal decomposition is preferably carried out at a temperature that is below the temperature where the metal or alloy melts or readily fuses, coalesces, or aggregates into larger particles. The removal of the ionic liquid decomposition products can be carried out in a rapid manner, such that any nanoparticles that might otherwise coalesce or aggregate can not do so in the short time period of decomposition and removal of the ionic liquid. The thermal decomposition can be carried out under a non-oxidizing atmosphere such as nitrogen or a noble gas such as argon. In this manner any undesired oxidation of the metal nanoparticle's surface can be avoided. When the nanoparticle cannot be readily oxidized, the decomposition can be carried out in air or other oxygen containing gas.

The inventive method can be readily adapted to prepare nanoparticle suspensions in a continuous or semicontinuous fashion rather than as a batch reaction as given in the example below. In the continuous process, the ionic liquid or liquids can be constantly introduced to a reactor with a constant physical vapor deposition of the metal or metals and the resulting suspension can be continuously removed from the reactor. The nature of the reactor can be modified as needed as is known by those of ordinary skill at the design of continuous reactors.

EXAMPLES

It should be understood that the Examples described below are provided for illustrative purposes only and do not in any way define the scope of the invention.

Example 1

Platinum nanoparticles of an average diameter of 1 nm suspended at 0.47 weight percent in 1-butyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)-imide (BMIM-Tf₂N), the ionic liquid, were prepared by sputtering platinum from a high purity (99.99+%) target into the ionic liquid in a stainless steel beaker rotated at a 45 degree angle containing a tetrafluoroethylene coated stirring bar which tumbled to agitate the liquid using a 2″ magnetron sputtering source and an argon plasma. The resulting suspension is illustrated in the scanned image from a scanning transmission electron microscope in FIG. 1.

In like manner to Example 1 a variety of suspensions of metal nanoparticles in ionic liquids were prepared. Table 1 lists the ionic liquids that were used, metal nanoparticle type, deposition conditions and weight loading in the ionic liquids. All of the nanoparticles remained stably dispersed in BMIM-Tf₂N for periods in excess of six months. Precipitation of material was observed for other ionic liquids after six months. Again, the STEM shown in FIG. 1 was performed after the Pt nanoparticle suspension in BMIM-Tf₂N had been stored for 21 months.

TABLE 1 Exemplary metal nanoparticle suspensions compositions and reaction conditions Weight Ar Deposition Deposition loading pressure Ionic liquid Metal time power (wt %) (mtorr) 1-butyl-3-methylimidazolium Gold  60 min 11 W 0.24 14.9 bis(triflouromethane sulfonyl)amide 1-butyl-3-methylimidazolium Cobalt  90 min 40 W 0.02 14.9 bis(triflouromethane sulfonyl)amide 1-butyl-3-methylimidazolium Cobalt 280 min 40 W 0.05 14.9 bis(triflouromethane sulfonyl)amide 1-butyl-3-methylimidazolium Nickel 102 min 40 W 0.03 14.9 bis(triflouromethane sulfonyl)amide 1-butyl-3-methylimidazolium Iron 216 min 40 W 0.05 14.9 bis(triflouromethane sulfonyl)amide 1-butyl-3-methylimidazolium Gold 170 min 11 W ~0.7 14.9 bis(triflouromethane sulfonyl)amide Triethylhydrogenammonium Platinum  60 min 21 W Unknown 14.9 bis(perfluoroethylsulfonyl)imide Triethylhydrogenammonium Platinum  60 min 21 W Unknown 14.9 bis(triflouromethane sulfonyl)amide 1-butyl-3-methylimidazolium Platinum 180 min 21 W 0.47 14.9 bis(triflouromethane sulfonyl)amide 1-butyl-3-methylimidazolium Manganese 198 min 30 W 0.0003 14.9 bis(triflouromethane sulfonyl)amide

It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples, which followed are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. 

1. A method for preparing a suspension of nanoparticles comprising the steps of: providing one or more ionic liquids; and depositing as a vapor at least one metal into said ionic liquid wherein metal containing nanoparticles become suspended in said ionic liquid.
 2. The method of claim 1, wherein said metal containing nanoparticles comprise a single metal.
 3. The method of claim 1, wherein said metal containing nanoparticles comprise an alloy.
 4. The method of claim 1, wherein said metal containing nanoparticles comprise a mixture of different metal nanoparticles.
 5. The method of claim 1, wherein said depositing step is carried out in an atmosphere of a reactive gas wherein the resulting nanoparticles comprise metal oxides, metal sulfides, metal nitride, metal oxysulfides, or metal oxynitrides.
 6. The method of claim 5, wherein said reactive gas is selected individually or in combination from the group consisting of oxygen, nitrogen, ammonia, and hydrogen sulfide.
 7. The method of claim 1, wherein said metal comprises platinum, silver, gold, cobalt, nickel, iron, manganese, rhodium, palladium, rhenium, ruthenium, iridium or osmium.
 8. The method of claim 1, wherein said ionic liquid comprises cations selected from the group consisting of: 1-alkyl-3-methylimidizolium

where R═C₁ to C₈, alkyl, N-alkyl pyridinium

where R═C₁ to C₈, alkyl, mono-, di-, tri- or tetraalkyl ammonium

where R=independently H, C₁ to C₈ alkyl, where at least one R≠H, and mono-, di-, tri- or tetraalkylphosphonium

where R=independently H, C₁ to C₈ alkyl and at least one R≠H, and anions selected from the group consisting of Cl⁻, Br⁻, I⁻, NO₃ ⁻, BF₄ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, CH₃(CH₂)_(x)CO₂ ⁻ where x=0to 18, and BR₄ ⁻ where R=independently C₁ C₈ alkyl.
 9. The method of claim 1, wherein the step of depositing comprises sputtering.
 10. The method of claim 1, further comprising the step of introducing a source of oxygen, sulfur or nitrogen at a sufficiently high temperature to the suspension of said nanoparticles comprising a metal, wherein said suspension is converted into a suspension of nanoparticles comprising metal oxides, metal sulfides, or metal nitrides.
 11. A method for preparing nanoparticles comprising the steps of: providing an ionic liquid; depositing as a vapor at least one metal onto said ionic liquid wherein metal containing nanoparticles are suspended in said ionic liquid; and heating said suspension to decompose said ionic liquid into gaseous neutral molecules wherein said remaining nanoparticles are essentially free of said ionic liquid, decomposition products of said ionic liquid, and other impurities.
 12. The method of claim 11, wherein said metal containing nanoparticles comprise a metal.
 13. The method of claim 11, wherein said metal containing nanoparticles comprise an alloy.
 14. The method of claim 11, wherein said metal containing nanoparticles comprise a mixture of different metal nanoparticles.
 15. The method of claim 11, wherein said step of depositing is carried out in an atmosphere of a reactive gas wherein the resulting nanoparticles comprise metal oxides, metal sulfides, metal nitride, metal oxysulfides, or metal oxynitrides.
 16. The method of claim 15, wherein said reactive gas is selected individually or in combination from the group consisting of oxygen, nitrogen, ammonia, and hydrogen sulfide.
 17. The method of claim 11, wherein said metal comprises platinum, silver, gold, cobalt, nickel, iron, manganese, rhodium, palladium, rhenium, ruthenium, iridium or osmium.
 18. The method of claim 11, wherein said ionic liquid comprise cations selected from the group consisting of 1-alkyl-3-methylimidizolium

where R═C₁ to C₈, alkyl, N-alkyl pyridinium

where R═C₁ to C₈, alkyl, mono-, di-, tri- or tetraalkyl ammonium

where R=independently H,C₁ to C₈ alkyl, where at least one R≠H, and mono-, di-, tri- or tetraalkylphosphonium

where R=independently H,C₁ to C₈ alkyl and at least one R≠H, and anions selected from the group consisting of Cl⁻, Br⁻, I⁻, NO₃ ⁻, BF₄ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, (CF₃SO₂)₂N⁻, CH₃(CH₂)_(x)CO₂ ⁻ where x=0to 18, and BR₄ ⁻ where R=independently C₁ to C₈ alkyl.
 19. The method of claim 11, wherein the step of depositing comprises evaporation, pulsed laser deposition or sputtering. 